Tandem mass spectrometry is a well known technique by which trace analysis and structural elucidation of samples may be carried out. In a first step, parent ions are mass analysed/filtered to select ions of a mass to change ratio of interest, and in a second step these ions are fragmented by, for example, collision with a gas such as argon. The resultant fragment ions are then mass analysed usually by producing a mass spectrum.
Various arrangements for carrying out multiple stage mass analysis or MSn have been proposed or are commercially available, such as the triple quadrupole mass spectrometer and the hybrid quadrupole/time-of-flight mass spectrometer. In the triple quadrupole, a first quadrupole Q1 acts as a first stage of mass analysis by filtering out ions outside of a chosen mass-to-charge ratio range. A second quadrupole Q2 is typically arranged as a quadrupole ion guide arranged in a gas collision cell. The fragment ions that result from the collisions in Q2 are then mass analysed by the third quadrupole Q3 downstream of Q2. In the hybrid arrangement, the second analysing quadrupole Q3 may be replaced by a time-of-flight (TOF) mass spectrometer.
In each case, separate analysers are employed before and after the collision cell. In GB-A-2,400,724, various arrangements are described wherein a single mass filter/analyser is employed to carry out filtering and analysis in both directions. In particular, an ion detector is positioned upstream of the mass filter/analyser, and ions pass through the mass filter/analyser to be stored in a downstream ion trap. The ions are then ejected from the downstream trap back through the mass filter/analyser before being detected by the upstream ion detector. Various fragmentation procedures, still employing a single mass filter/analyser, are also described, which permit MS/MS experiments to be carried out.
Similar arrangements are also shown in WO-A-2004/001878 (Verentchikov et al). Ions are passed from a source to a TOF analyser, which acts as an ion selector, from where ions are ejected to a fragmentation cell. From here, they pass back through the TOF analyser and are detected. For MSn, the fragment ions can be recycled through the spectrometer. US-A-2004/0245455 (Reinhold) carries out a similar procedure for MSn but employs a high sensitivity linear trap rather than a TOF analyser to carry out the ion selection. JP-A-2001-143654 relates to an ion trap, ejecting ions on a circular orbit for mass separation followed by detection.
The present invention seeks against this background to provide an improved method and apparatus for MSn.